1. Field of the Invention
The present invention relates to Doppler sonar apparatus, and more specifically to marine Doppler sonar apparatus for use in deep water.
2. Description of the Prior Art
Marine Doppler sonar apparatus for navigation, speed measurement, and the like is well known in the art. In general, such apparatus includes means for projecting an acoustic beam angularly downward toward the ocean bottom. Energy reflected from the bottom returns to the sonar apparatus in which it is analyzed and processed to obtain the desired information.
In relatively shallow water, the reflected signal is sufficiently strong to permit accurate analysis. However in deep water, the signal is highly attenuated so that the signal reaching the sonar apparatus is inadequate to provide accurate measurements.
One means for providing usable measurements in deep as well as shallow water is disclosed in U.S. Pat. No. 3,795,893 issued to J. A. Kritz and S. D. Lerner. In this apparatus, signals are reflected from the bottom in relatively shallow water. In deeper water, gating means are used to sample reflections arising from discontinuities present in the sea water at specified distances from the vessel. U.S. Pat. No. 3,795,893 also illustrates a "Janus" type installation wherein pairs of acoustic beams are launched angularly toward the ocean bottom so as to have components in opposite horizontal directions. Horizontal motion of the ship in the plane of the beams is detected by an increase in frequency in the reflected signal arising from one beam, and a decrease in frequency in the other reflected signal.
Alternatively, parametric arrays have been proposed for use in deep water. The basic theory for parametric operation was described, for instance, by P. J. Westerveldt in an article appearing in Vol. 35 of the Journal of the Acoustical Society of America in April, 1963 and entitled: "Parametric Acoustic Array." In this article, Westerveldt proposed launching difference frequency waves by two highly collimated sound beams. According to Westerveldt's theory, the absorptive properties of the propagation medium would attenuate the high frequency waves and permit the resultant low frequency wave to propagate in an area remote from the source.
In a second article in Vol. 55 of the same Journal, dated January 1974, and entitled "On the Performance of a Dual Frequency Parametric Source," F. H. Fenlon presented a theoretical discussion relating to the formation of a farfield difference frequency signal resulting from the non-linear interaction in the propagation medium when two different frequency primary waves are radiated simultaneously by a finite amplitude source. Various arrangements for providing practical parametric arrays for marine use have been tried. In some of these designs, an acoustic converging lens is used to collect reflected acoustic energy and focus this energy on transducers placed in the focal plane of the lens. However, in such an arrangement, the transducers should be made relatively small in order to be fully irradiated by the concentrated sound waves. If such transducers are then used to generate the required high power acoustic wave, such transducers are subject to extreme cavitation and resultant failure because of their small radiating area. Furthermore, the simultaneous production of multiple high power beams can cause cavitation in the region of beam intersection.
The present invention provides means to overcome these shortcomings.